Digital imaging systems such as digital cameras utilize semiconductor chips equipped with photo-sensitive electronic components, such as photo-diodes. The digital imaging systems typically capture light information in a series of pixels. Commonly, the pixels are arranged in an array of rows and columns, such as 1024×768 pixels. Each pixel is represented by at least one photo-sensitive component. In applications requiring the capture of color, color filters may be used to capture the specific colors of the received light, and each pixel may be represented by more than one photo-sensitive component.
Generally, a microlens guides the light to the photo-sensitive component, essentially acting as the collection point for the digital imaging system. A microlens is a tiny lens formed on a semiconductor chip above a photo-sensitive component. Because the collected light passes through the microlens, it is important that the microlens be shaped such that it accurately guides light to the photo-sensitive component.
Microlenses are generally formed by applying a layer of microlens material on a semiconductor chip. The microlens material, which is typically a photo-resist material, is exposed in accordance with a desired pattern and developed to remove unwanted microlens material. After the microlenses have been patterned, a reflow process is performed to cause the microlenses to form a substantially uniform symmetrical lens shape.
Problems, however, may be incurred during the fabrication of the microlenses as the design sizes are reduced. For example, as the design size is reduced, the microlenses are positioned closer together. As the microlenses are positioned closer together, the microlenses have a tendency to merge during the reflow procedure. The reflow procedure, which uses high temperatures, is difficult to control and, thus, it is difficult to prevent the microlenses from merging as the distance between microlenses decreases.
Another problem often seen is related to the focal length. Many times the thickness of the device will increase due to, among other things, additional layers (e.g., metal layers) as more circuitry is integrated onto a single chip. The additional layers in turn increase the focal length, which requires a thinner microlens. The reflow procedure to create thinner microlenses generally uses a higher temperature, which further increases the possibility of the microlenses merging.
Therefore, there is a need for a method for fabricating microlenses to reduce the critical dimension between microlenses and to reduce the thickness of the microlenses.